Currently, a mobile telephone terminal and mobile telephone network using the third generation W-CDMA (Wideband Code Division Multiple Access) technology realize voice telephony and TV telephone services by using a circuit switching network or circuit switching protocol.
Meanwhile, there is a trend in which these multimedia services such as voice and TV telephone services are realized on an IP network using IMS (IP Multimedia Subsystem), and it is more likely that these services are integrated on the IMS network in the future. For the configuration of IMS, one can refer to 3GPP (Third Generation Partner Project) TS23.002 and 3GPP 23.228.
Due to research and development on high-speed and large capacity technologies for mobile networks, the speed and capacity of mobile networks based on IP have increased and as of now these networks have been implemented: HSDPA (High Speed Downlink Packet Access), HSPA (High Speed Packet Access), EVDO Rev. A, and EVDO Rev. B.
In the future, transmission at even higher speed will be possible, and LTE (Long Term Evolution), EPC (Evolved Packet Core), UMB (Ultra Mobile Broadband) and Mobile WiMax aiming at downlink peak rates of at least 100 Mbps (Mega bits per second) and uplink rates of at least 50 Mbps will be introduced.
Further, it is predicted that voice telephony will evolve into VoIP (Voice over IP) based on IP in these transmission schemes.
Patent Document 1 discloses a configuration in which a mobile terminal switches from a first media gateway MG1 to a second media gateway MG2 during a handover; after the handover, an anchor media gateway MG0 calculates the difference between a reference value received from the second media gateway MG2 and a reference value that was received from the first media gateway when no handover occurred; this difference is then returned to the second media gateway MG2 to synchronize its series therewith; and the second media gateway MG2 receives the difference and changes the reference value. In Patent Document 1, the first and the second media gateways MG1 and MG2 and the anchor media gateway MG0 communicate on an IP network.
Patent Document 2 discloses the configuration of a mobile communication system in which a mobile switching center is enabled to identify the order of signals sent by a mobile station by adding a multi-frame number on a radio channel to a packet header transmitted by a radio base station to the mobile switching center, thereby avoiding to mistake the order of information transmitted from the mobile switching center to a public network upon handover. In this mobile communication system, the mobile switching centers, to which the radio base stations are connected, provide serial numbers to information of time slots in frames of downlinks transmitted from two adjacent radio base stations to one mobile station, and handover processing is performed based on these serial numbers.